The ORIGIN HTTP/2 Frame

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119].

The ORIGIN frame type is 0xb (decimal 11).

+-------------------------------+-------------------------------+
|         Origin-Len (16)       | ASCII-Origin? (*)           ...
+-------------------------------+-------------------------------+

Origin-Len:

An unsigned, 16-bit integer indicating the length, in octets, of the ASCII-Origin field.

Origin:

An optional sequence of characters containing the ASCII serialization of an origin ([RFC6454], Section 6.2) that the sender believes this connection is or could be authoritative for.

The ORIGIN frame does not define any flags. However, future updates to this specification MAY define flags. See Section 2.2.

The ORIGIN frame is a non-critical extension to HTTP/2. Endpoints that do not support this frame can safely ignore it upon receipt.

When received by an implementing client, it is used to initialise and manipulate the Origin Set (see Section 2.3), thereby changing how the client establishes authority for origin servers (see Section 2.4).

The origin frame MUST be sent on stream 0; an ORIGIN frame on any other stream is invalid and MUST be ignored.

Likewise, the ORIGIN frame is only valid on connections with the “h2” protocol identifier, or when specifically nominated by the protocol’s definition; it MUST be ignored when received on a connection with the “h2c” protocol identifier.

This specification does not define any flags for the ORIGIN frame, but future updates might use them to change its semantics. The first four flags (0x1, 0x2, 0x4 and 0x8) are reserved for backwards-incompatible changes, and therefore when any of them are set, the ORIGIN frame containing them MUST be ignored by clients conforming to this specification. The remaining flags are reserved for backwards-compatible changes, and do not affect processing by clients conformant to this specification.

The ORIGIN frame is processed hop-by-hop. An intermediary MUST NOT forward ORIGIN frames. Clients configured to use a proxy MUST ignore any ORIGIN frames received from it.

Each ASCII-Origin field in the frame’s payload MUST be parsed as an ASCII serialisation of an origin ([RFC6454], Section 6.2). If parsing fails, the field MUST be ignored.

Senders should note that, as per [RFC6454] Section 4, the values in an ORIGIN header need to be case-normalised before serialisation.

• a scheme of “https”,
• a host that is reflected in a subjectAltName of the connection’s TLS certificate (using the wildcard rules defined in [RFC2818], Section 3.1), and
• a port that reflects the connection’s remote port on the client.

If any of these requirements are violated, the client MUST ignore the field (but still continue processing the frame).

See Appendix A for an illustrative algorithm for processing ORIGIN frames.

The set of origins (as per [RFC6454]) that a given connection might be used for is known in this specification as the Origin Set.

• Scheme: “https”
• Host: the value sent in Server Name Indication ([RFC6066] Section 3), converted to lower case
• Port: the local port of the connection on the server

The contents of that ORIGIN frame (and subsequent ones) allows the server to incrementally add new origins to the Origin Set, as described in Section 2.2.

The Origin Set is also affected by the 421 (Misdirected Request) response status code, defined in [RFC7540] Section 9.1.2. Upon receipt of a response with this status code, implementing clients MUST create the ASCII serialisation of the corresponding request’s origin (as per [RFC6454], Section 6.2) and remove it from the connection’s Origin Set, if present.

[RFC7540], Section 10.1 uses both DNS and the presented TLS certificate to establish the origin server(s) that a connection is authoritative for, just as HTTP/1.1 does in [RFC7230]. Furthermore, [RFC7540] Section 9.1.1 explicitly allows a connection to be used for more than one origin server, if it is authoritative. This affects what requests can be sent on the connection, both in HEADERS frame by the client and as PUSH_PROMISE frames from the server.

• Clients MUST NOT consult DNS to establish the connection’s authority for new requests. The TLS certificate MUST be used to do so, as described in [RFC7540] Section 9.1.1.
• Clients sending a new request SHOULD use an existing connection if the request’s origin is in that connection’s Origin Set, unless there are operational reasons for creating a new connection.
• Clients MUST use the Origin Set to determine whether a received PUSH_PROMISE is authoritative, as described in [RFC7540], Section 8.2.2.

Note that this does not prevent clients from performing other certificate checks as required or allowed, either at connection time or when processing ORIGIN. See [RFC7540] Section 9.1.1 for more information.

Because ORIGIN can change the set of origins a connection is used for over time, it is possible that a client might have more than one viable connection to an origin open at any time. When this occurs, clients SHOULD not emit new requests on any connection whose Origin Set is a subset of another connection’s Origin Set, and SHOULD close it once all outstanding requests are satisified.